Ultra-thin films of organic molecules are conventionally prepared by a spin-coating technique, an electrolytic polymerization technique, a vapor deposition technique, a vapor deposition polymerization technique or the like. In addition, Langmuir-Blodgett (LB) method is well known as a method for obtaining an alignment layer. According to this method: amphiphilic molecules are dissolved in a volatile organic solvent and deposited at a gas-liquid interface; the solvent is evaporated to be compressed; and the resultant monomolecular layer is transferred onto a solid substrate. This method allows the control of the number of the thin film layers and the order of the laminated layers. Also known are a method in which polymerizable functional groups are introduced into amphiphilic molecules to form an ultra-thin film by LB method which is then polymerized for stabilization, and a method in which an ultra-thin film is obtained by LB method from already polymerized high-molecular-weight amphiphilic molecules or amphiphilic block copolymers.
Moreover, for a free-standing ultra-thin polymer film having an arbitrary shape, for example, a method in which a self-assembled monomolecular layer is formed on a gold substrate that has a pattern acquired by microlithography technique, then polymerizable molecules are adsorbed and polymerized in water and the formed ultra-thin polymer film is peeled off from the gold substrate, and a method in which polymer electrolytes are alternatively laminated on a substrate to form an ultra-thin polymer film and then the ultra-thin film is peeled off from the substrate by using an aqueous support membrane to prepare an ultra-thin film having the same size as the substrate are known (see, for example, Patent Document 1: WO 2006/025592, Patent Document 2: WO2008/050913, etc.).
Meanwhile, a composite membrane made of a plurality of polymers or block copolymers is known to have a micro-phase-separated structure which includes spherical, columnar, lamellar and gyroidal structures. For example, a method in which a micro-phase-separated columnar structure formed with an amphiphilic block copolymer is utilized so as to decompose and eliminate the polymer forming the columns by means of plasma, light, an electron beam, heat, an acid, a base, a reductant or the like, thereby obtaining a porous film is known (see, for example, Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-155365, Patent Document 4: Japanese Unexamined Patent Application Publication (Translation of PCT) No. 2004-502554, Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-124088, Patent Document 6: Japanese Unexamined Patent Application Publication No. 2010-116463, etc.).
When an amphiphilic polymer solution is applied onto a glass substrate or the like and humid air is sent upon preparing a cast film, latent heat is released upon evaporation by which the moisture builds up condensation, leaving homogeneous array of droplets on the solution. According to a known method, these self-assembled droplets serve as templates to give clearly opened pores with a constant size of few microns in a polymer film, thereby forming a thin polymer film having a honeycomb structure (see, for example, Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-70254, etc.).
Since porous films that utilize such an above-described micro-phase-separated structure use polymers having a particular kind of structure such as block copolymers or amphiphilic polymers, they lacked versatility. Furthermore, use of a general polymer for obtaining a porous ultra-thin polymer film having a plurality of pores in a free-standing ultra-thin organic polymer film has been unknown. In addition, a method for producing such a porous ultra-thin polymer film has also been unknown.
Here, claim 1 of Patent Document 8 states a method for producing a porous film, comprising the steps of: applying an application liquid containing an organic compound and a hydrophobic organic solvent onto a support to form a coating layer; and condensing water vapor on the coating layer to dry the coating layer (i.e., condensing and drying steps).
In addition, claims 1 of Patent Documents 9 and 10 state a porous film comprising a micro-phase-separated structure including a continuous phase having a water-insoluble polymer A as the primary component and a cylindrical microdomain having a water-soluble polymer B as the primary component, wherein cylindrical micropores with an average pore diameter of 1-1000 nm are present in the cylindrical microdomain.
Moreover, Example 2 of Patent Document 11 describes that a substance resulting from PMMA deterioration is removed from an asymmetric diblock/copolymer film made of polystyrene (PS) and polymethyl methacrylate (PMMA) formed on a gold film to prepare a PS nanopore template.